1. Field of the Invention
This invention relates to a rotation angle sensor which uses giant magnetoresistive elements showing a large change in resistance in accordance with a change of an external magnetic field.
2. Description of the Related Art
A conventional rotation angle sensor will now be described with reference to FIG. 8. In the conventional rotation angle sensor, each of giant magnetoresistive elements RA, RB formed on a board (not shown) has a multilayer body C5 which is constituted by an antiferromagnetic layer 51, a fixed magnetic layer 52 stacked on the antiferromagnetic layer 51 and having its direction of magnetization fixed by the antiferromagnetic layer 51, a non-magnetic conductive layer 53, and a free magnetic layer 54 facing the fixed magnetic layer 52 with the non-magnetic conductive layer 53 held between them, and also has a pair of electrodes (not shown) holding the multilayer body C5 from both sides. The multilayer body C5 has a rectangular planar shape and its length in the long-side direction is approximately several mm.
Such giant magnetoresistive elements RA, RB are provided to form a pair so that their fixed magnetic layers 52 have opposite directions of magnetization (directions e in FIG. 8). The pair of giant magnetoresistive elements RA, RB are electrically connected with each other so that a differential is outputted with respect to the electric resistance value between the electrode layers.
A rotating part (not shown) facing the board has a magnet which is magnetized as the north pole and the south pole on the side facing the board. The magnetized side is rotatable facing the pair of giant magnetoresistive elements RA, RB. As the rotation of the magnet causes a magnetic field applied to the giant magnetoresistive elements RA, RB to rotate within a plane parallel to the board on which the giant magnetoresistive elements RA, RB, the direction of the magnetization of the free magnetic layer 54 in the giant magnetoresistive elements RA, RB changes in accordance with the direction of the magnetic field of the rotating magnet.
The electric resistance value between the electrode layers of the giant magnetoresistive elements RA, RB is reduced when the direction of magnetization of the free magnetic layer 54 is coincident with the direction of magnetization of the fixed magnetic layer 52. The electric resistance value is raised when the direction of magnetization of the free magnetic layer 54 is opposite to the direction of magnetization of the fixed magnetic layer 52. In this case, since the fixed magnetic layers 52 of the giant magnetoresistive elements RA, RB have opposite directions of magnetization to each other, the giant magnetoresistive elements RA, RB have opposite changes in electric resistance value due to the rotation of the magnet. With respect to a pair of such giant magnetoresistive elements RA, RB, a magnetic noise component due to a change in magnetic field environment or the like is eliminated from the differential output related to the electric resistance value.
The differential output based on the change in electric resistance value of the pair of giant magnetoresistive elements RA, RB can be found as a function sinxcex8 with xcex8 being a variable, where xcex8 represents the rotation angle of the magnet to the giant magnetoresistive elements RA, RB.
However, in such a conventional rotation angle sensor, since the multilayer bodies C5 of the giant magnetoresistive elements RA, RB are large, the magnet having a length of approximately several cm between both poles causes the magnetic field generated by the magnet to be strained at the ends of the multilayer bodies C5 and a uniform magnetic field cannot be applied to the multilayer bodies C5, making it difficult to accurately detect the rotation angle xcex8. Moreover, the giant magnetoresistive elements R of such a large size have a disadvantage of high material cost.
If a magnet which can apply a uniform magnetic field to the multilayer bodies C5 is increased in size, the rotation angle sensor is increased in size. Moreover, if the size of the multilayer bodies C5 of the giant magnetoresistive elements RA, RB is reduced to 1 mm or less to enable uniform application of a magnetic field to the multilayer bodies C5 by the magnet, the differential output of the electric resistance value is deviated from the function of sinxcex8 and the angle xcex8 cannot be detected accurately.
It is an object of the present invention to provide a rotation angle sensor which can accurately detect the rotation angle with its output accuracy improved without increasing the size.
A rotation angle sensor according to the present invention comprising: a fixed part on which giant magnetoresistive elements are provided, each of the giant magnetoresistive elements having a multilayer body including a fixed magnetic layer with a fixed direction of magnetization, a non-magnetic conductive layer, and a free magnetic layer facing said fixed magnetic layer with said non-magnetic conductive layer held between them, and electrode layers connected both ends of said multilayer body; and a rotating part which forms a magnetic field and rotates facing said fixed part,
wherein said giant magnetoresistive elements with their fixed magnetic layers having opposite directions of magnetization are provided as a pair on a surface of said fixed part, and said giant magnetoresistive elements as a pair are connected so that a difference between their resistance values is outputted, thus detecting a magnetic field component parallel to the surface of said fixed part provided from said rotating part to said giant magnetoresistive elements,
wherein the size of said multilayer body of said giant magnetoresistive element is 1 mm or smaller, and when the shape anisotropy energy (H) of said free magnetic layer is expressed by H=A*Js*(Br/xcfx80), where Js represents saturation magnetization of a magnetic material forming said free magnetic layer and Br represents remanent magnetic flux density of the magnetic material forming said free magnetic layer, xe2x88x92350 less than A less than 0 holds.
Such a rotation angle sensor is small and can accurately detect the rotation angle since a uniform magnetic field can be applied to the multilayer bodies without using a large magnet. Moreover, as the shape anisotropy energy of the free magnetic layer is relaxed, the output can be expressed substantially by a function sin xcex8 with respect to a rotation angle xcex8 and the rotation angle can be accurately detected. Magnetostriction is almost zero, and when the giant magnetoresistive elements constitute a Wheatstone bridge circuit, the output has a deviation of 1% or less from the function sinxcex8 with respect to the rotation angle xcex8 in comparison with an input voltage to the Wheatstone bridge circuit.
In the rotation angle sensor according to the present invention, said free magnetic layer has a thickness of 1.5 to 8 nm.
In such a rotation angle sensor, since the rate of change in resistance based on a GMR effect of the giant magnetoresistive elements is large and magnetization of the free magnetic layer tends to be in the direction of the magnetic field, the output accuracy is improved further and high output sensitivity is realized.
In the rotation angle sensor according to the present invention, said rotating part has a magnet and the maximum size of the magnet in the direction of the magnetic field is five times or more of the size of the GMR element in the long-side direction of said multilayer body.
In such a rotation angle sensor, since a magnetic field generated by the magnet is uniformly applied to the multilayer bodies of the GMR elements, the output accuracy can be improved further.
In the rotation angle sensor according to the present invention, the magnetic field applied to said giant magnetoresistive elements from said rotating part has a magnitude not less than the magnitude of the saturation magnetization of said free magnetic layers.
In such a rotation angle sensor, since the magnetic fields of the free magnetic layers is equalized by the magnetic field generated by the magnet, it is not necessary to form a layer for equalizing the magnetic fields of the free magnetic layers.
The rotation angle sensor of the present invention is small and can detect the rotation angle accurately because a uniform magnetic field can be applied to the multilayer bodies without using a large magnet. Moreover, since the shape anisotropy energy of the free magnetic layer is relaxed, when the rotation angle of the magnet with respect to the giant magnetoresistive elements is xcex8, an output is expressed by a function sin xcex8 with respect to the rotation angle xcex8 and the rotation angle xcex8 of the magnet can be accurately detected.